Problem: The zoo got $18$ new animals. $3$ of them are lions and the rest are monkeys. How many new monkeys did the zoo get?
Answer: We can solve this problem in two ways: $ \begin{aligned}&{3} \text{ lions}\\ +& \underline{{?} \text{ monkeys}}\\ =&{18} \text{ new animals}\end{aligned}$ Or: $ \begin{aligned}&{18} \text{ new animals}\\ -& \underline{{3} \text{ lions}}\\ =&{?} \text{ monkeys}\end{aligned}$ ${15}$ monkeys